This invention relates to a combustion apparatus, which is equipped with an atomizer, applicable to the combustor of, e.g., a fan heater, boiler, hot water supply device, drier or gas turbine. The invention also relates to a method of controlling the combustion apparatus.
Since a combustor used in a variety of heating apparatus such as oil stoves and fan heaters and in a variety of driers generally is placed in an office or in a residential building such as a private home, various measures have been tried for the purpose of preventing living spaces from becoming contaminated or malodorous. For example, a system has been developed in which the combustor is provided with heating means such as a heater for gasifying the externally supplied fuel. However, it has become clear that some time (at least 30-40 seconds, and as long as 2-3 minutes in many cases) is needed for the fuel to gasify, and that some time therefore is also required for the fuel to ignite.
In order to solve these problems, the applicants amongst others have proposed various systems for the purpose of shortening ignition time by hastening the speed at which the gas of vaporized fuel is produced within a combustion chamber. Specifically, in a proposed arrangement, an atomizer, e.g., an ultrasonic atomizing device comprising a high-frequency oscillator, an electroacoustic transducer and an ultrasonic oscillator horn is provided at the lower portion of a combustion chamber. By exciting this ultrasonic atomizing device into operation, the fuel supplied to the combustion chamber is atomized into very fine droplets, thereby accelerating vaporization of the fuel in the combustion chamber and, hence, reducing the time needed for steady combustion of the fuel.
However, combustion tests using this conventional combustor having the ultrasonic atomizing device provided at the lower portion of the combustion chamber have shown that while the time required for combustion at the lower portion (namely the vaporizing compartment) of the combustion chamber having the ultrasonic atomizing device is indeed greatly curtailed in comparison with the prior art, the flame produced in the vaporizing compartment travels to the overlying combustion chamber, where a steady-state flame is formed. It has been confirmed that some time, on the order of 10-30 seconds, is required for the steady-state flame to be formed from the moment of fuel ignition in the vaporization compartment, and that odors and soot are produced to some degree until the steady-state flame is formed. Furthermore, in this proposed combustion apparatus equipped with the ultrasonic atomizing device, the structure of the combustor itself is complicated, and it has been clarified that the turn-down ratio (the ratio of maximum amount of combustion to minimum amount of combustion), which represents the maximum adjustment range of the amount of combustion, can be set to only 2:1.
In addition, though the combustion apparatus with the atomizer can be applied to a so-called line-type combustor the entirety of which generally has the shape of a rectangular parallelepiped, and though this arrangement greatly contributes to a more slender apparatus, a problem encountered is that a blue flame cannot be maintained at the end of the flame hole.
In the prior-art combustion apparatus with the atomizer, droplets form and attach themselves to the wall surface of the vaporization compartment when ignition is delayed by low temperatures. These droplets grow, flow downwardly and drop onto the combustion plate, clogging its air holes. The result is a yellowish flame of extended length. This causes a disturbance in the combustion process and leads to problems in terms of achieving stable combustion.
Since the conventional combustion apparatus has a combustion space formed in only two sides or, in some cases, in one side, the apparatus must be increased in size in order to enlarge the combustion space.
In the case of the so-called line-type combustor the entirety of which generally has the shape of the rectangular parallelepiped, the combustion space is long and narrow. Consequently, the side walls of a spray chamber thermally expand due to the heat of combustion and the atomization chamber becomes deformed. As a result, the flame distribution is disturbed, combustion becomes non-uniform and a red flame is produced.
Furthermore, since the combustion space is long and narrow, air which is injected from air supply holes provided in a wind box is non-uniform at the entrance and inner confines of an air passageway. As a result, a disturbance is produced in the distribution of the flame. This leads to non-uniform combustion and the production of a red flame, which is undersirable. Furthermore, another problem is that when only a small amount of secondary air flows in the steady state, unburned gas flows backwardly into a secondary air duct, causing tar deposits to form.
When the flame is extinguished in the conventional combustion apparatus, some of the fuel which has been supplied up to the distal end portion of a fuel supply pipe facing the oscillator horn of the atomizer drips into the spray chamber via the oscillator horn due to thermal expansion and a drop in surface tension and viscosity caused by heating several seconds after the fuel pump is turned off. This phenomenon is referred to as flashback and results in incomplete combustion and the formation of soot. The unburned raw gas produced is discharged and leads to the formation of tar that deposits on the oscillator horn. The end result is a decline in the fuel atomization efficiency and the production of odors.
Other problems that remain involve the uniformity of flame distribution, a rise in the temperature of the spray chamber and the exhaust gas characteristics at the time of incomplete combustion.